The purification and filtration of water and other aqueous solutions to remove toxic materials, such as selenium, is necessary for many applications, such as the treatment of feeds, waste streams, process streams and by-products associated with various industrial processes, the provision of safe portable drinking water, and the treatment and control of municipal waste water. The presence of various compound forms of selenium in water is of great environmental concern as selenium-containing compounds can be extremely toxic. Additionally, regulatory agencies have placed strict guidelines on discharging selenium to the environment. In particular, facilities that generate selenium are restricted to 12 parts per billion (ppb) discharge limits and in many cases must treat to less than 5 ppb.
In aqueous environments or water, dissolved inorganic Se is normally present as (+6 oxidation state) selenate (SeO4−2) and as (+4 oxidation state) selenite (SeO3−2). The soluble inorganic Se forms, selenite and selenate, account for the majority of the total Se concentration found in natural, agricultural, or discharged waters and other aqueous solutions. The proportion of selenate/selenite present in waters is generally governed by the pH-redox status of the system. Selenate is stable under alkaline and oxidizing conditions and selenite is stable under mildly oxidizing conditions. The ratio of selenate to selenite present in natural waters is also affected by the different adsorption kinetics of selenate versus selenite. Selenite has a strong affinity for a variety of common minerals at pH values less than 7, whereas selenate does not; selenite also has a strong affinity for particulate organic matter. Constituents adsorbing selenite include Al and Fe oxides, clay minerals, and calcite. Also some microbial populations selectively assimilate selenite over selenate. Due to the many available mechanisms for selenite removal from waters, selenate is the major soluble Se species that remains in treated waters.
Known methods for purifying aqueous solutions to remove selenium that take advantage of such behaviors/properties (of selenite) include, for example, reverse osmosis, distillation, ion-exchange, chemical adsorption, coagulation, flocculation, and filtering or retention. However, with respect to removal of selenate, many of these purification practices are ineffective and/or inefficient (in terms of time, energy and added waste), and require significant technical know-how and sophistication to implement.
For example, chemical coagulants such as ferric oxide are cheap and effective at removing selenite when fed in excess, but large amounts of sludge are also generated and show little efficacy for removing selenate. Activated alumina has also been shown to be an effective media for removing selenite, but gives poor results for selenate removal. While biological reactors have been shown to remove both selenite and selenate by reduction to elemental selenium, these systems cost around $30 million to treat 300-400 gpm (gallon per minute) of water. Additionally, microbiological upsets occur causing selenium levels to spike. As a result, many advanced fluid purification technologies have had limited application with respect to removal and collection of selenate (itself) from waters and other aqueous solutions.
There remains a need for improved removal methods (in terms of, for example, cost, simplicity, reliability, efficiency and selectivity) for removal of selenate from water. The improved methods would desirably contain an active composition stable to decomposition (i.e., a composition that neither bleeds into surrounding water, nor decomposes to form a harmful substance). The disclosure that follows describes such methods and compositions for removing selenium containing ions, such as selenate, from water.